Many optical and/or opto-electronic devices depend for the operation on the presence of a non-equilibrium (with respect to the device temperature) density of electrical carriers (electrons and/or holes) in at least a portion of the device, with the refractive index of the material depending on the density of carriers. Typically the relevant portion of the device consists of semiconductor material, and the non-equilibrium carrier distribution is created by the absorption of electromagnetic radiation (resulting in the creation of electron/hole pairs). However, the non-equilibrium distribution can also be produced by injection of carriers into the portion of the device, e.g., by means of a p-n junction, as will be apparent to those skilled in the art.
However created, the rate at which the non-equilibrium carrier distribution can decay affects the speed at which the device can be operated, e.g., the minimum time between two signal pulses to which the device can respond. It is obviously desirable that the speed of operation be high, and thus it is important to have available means that can affect a speedy decay of a non-equilibrium carrier distribution in a relevant portion of an optical or opto-electronic device. This application discloses such means.
Furthermore, in many semiconductor-based optical and/or opto-electronic devices, an important mechanism for the decay of a non-equilibrium carrier distribution is non-radiative pair recombination. As is well known, this mechanism results in heating of the device, since the energy given up by the electron/hole pair is transferred to the lattice. This frequently poses difficult heatsinking problems, which may, for instance, limit the possible area density of certain devices. e.g., integrated optical switches or logic elements. Thus it would be highly desirable to have available means that can effect a shift away from non-radiative recombination towards radiative recombination, since in the latter at least a portion of the energy given up by a carrier pair is removed from the device in the form of an emitted photon, resulting in eased heat-sinking requirements. This application also discloses such means.
Although the invention can be embodied in a variety of devices, including radiation detectors (an opto-electronic device), much of the discussion below will, for purposes of ease of exposition, be in terms of a particular class of optical devices, namely, nonlinear devices comprising a Fabry-Perot-type etalon. No limitation is thereby implied.
Bistable and other nonlinear optical devices have been known for some time, and a wide variety of signal processing functions can be carried out by means of bistable devices ("bistable" and "nonlinear" are used herein interchangeably unless indicated otherwise by the context). A recent monograph, H. M. Gibbs, Optical Bistability: Controlling Light With Light, Academic Press (1985) can serve as an introduction to the field of bistable optical devices. For instance, on pages 1-17, incorporated herein by reference, are given brief discussions of bistable optical logic devices (both two-state and many-state), of an optical transistor, of optical discriminators, limiters, pulse compressors, oscillators, gates, and flip-flops. Pages 195-239, also incorporated herein by reference, contain a detailed discussion of optical switching.
Many of the nonlinear optical devices comprise a nonlinear Fabry-Perot (FP) etalon, a fixed-spacing optical cavity with, typically an optically nonlinear medium within the cavity. Furthermore, much of the work on optically nonlinear devices has focused on devices using solid (typically semiconductor, mostly GaAs-based) nonlinear media. Such media are, for instance, homogeneous GaAs, and GaAs-AlGaAs multiple quantum well (MQW) structures.
In U.S. patent application Ser. No. 870,842, filed June 5, 1986, incorporated herein by reference, is disclosed a monolithic Fabry-Perot etalon with active multilayer mirrors that can be produced by known deposition and patterning techniques without any critical etching step. These etalons can have high finesses, and can be produced in the form of a multi-etalon arrays.
A principal limitation on the operational speed of an optical device that comprises a nonlinear etalon is the recombination time of the hole-electron pairs created in the nonlinear spacer material of the device. As will be readily understood by those skilled in the art, the density of pairs in the relevant portion of the device has to decrease to a relatively small value (from the relatively high value required for the nonlinear action to occur) before another switching action can be initiated.
Surface recombination is a known means for speeding the recovery of GaAs etalons. See, for instance, Y. H. Lee et al., Applied Physics Letters, 49,486 (1986). Such recombination typically is nonradiative, releasing essentially all of the energy as heat. Furthermore, due to the relatively long distances involved in the diffusion of hole/electron pairs to the surfaces of typical devics, surface recombination is expected to be limited in the recovery speed-up it can produce. Exemplarily, it may be difficult to obtain recovery times less than about 30 ps with prior art structures.
Due to the promise held by nonlinear FP etalons (as well as by other optical or opto-electronic devices that depend for their functioning on the temporary presence of a non-equilibrium carrier distribution), for instance, in the field of optical data processing (including optical computing), and in optical communications, it would be highly desirable to have available means for speeding up recovery of the device which, optionally, can increase the ratio of radiative to non-radiative recombination, thereby easing heat-sinking requirements.
For information on optical computing, see Proceedings of the IEEE, Vol. 72(7) 1984, especially A. A. Sawchuck et al., (pp. 758-779), and H. Huang (pp. 780-786). A. Huang et al., Proceedings of the IEEE Global Telecommunications Conference, Atlanta, Ga., 1984, pp. 12114 -125 discloses telecommunications apparatus that can be implemented using nonlinear optical devices.